Scratch-resistant mask for photolithographic processing

ABSTRACT

Disclosed is a photomask and a method for the manufacture thereof. A metal stencil is disposed on a glass substrate such that only preselected areas of the glass which are to be made opaque are exposed. A grit-etch step follows in which depressions are formed in the glass substrate in the preselected areas. Fusible masking material is sprayed in the depressions through the metal stencil. The masking material is preferably in particulate form in a volatile carrier liquid. The carrier liquid is preferably first driven off and, then, the combination is exposed to a relatively high temperature that fuses the masking material to the substrate.

BACKGROUND OF THE INVENTION

This invention relates generally to photolithographic masks and, moreparticularly, to masks used in processes such as semiconductor devicemanufacture.

The manufacture of semiconductor devices usually entails severalphotomasking steps to define and isolate portions of semiconductorsurfaces in diffusion, etching and plating processing. Generally, thesurface of a semiconductor wafer is covered with photoresist which is inturn covered with a photolithographic mask (hereinafter referred to as aphotomask). The combination is then exposed to light and the regions ofthe photoresist which are exposed by the mask are polymerized. Thephotomask performs the function of transmitting the light in selected,well-defined areas while blocking light from the remainder of the areas,thus causing a photomask-patterned latent image to be formed in thephotoresist layer. During exposure, the photomask is usually located inintimate juxtaposition or even in direct contact with thephotoresist-coated surface. Next, the combination is exposed to areagent which dissolves the non-polymerized photoresist but to which thepolymerized photoresist is resistant. The remaining photoresist,comprising an image of the photomask, then serves a purpose; such as,for example, selectively protecting underlying material from attack byliquid or gaseous etchants.

Conventional photomasks used in semiconductor processing are generallyformed by selectively locating a photographic emulsion on a glasssubstrate. A problem encountered with such masks is that the relativelysoft emulsion is easily scratched in handling or when using, thusshortening the period of utility of the photomask. Efforts to solve thisproblem have been made in the past. For example, masks were fabricatedby fusing hard, glassy masking material to a glass substrate, see U.S.Pat. Nos. 3,743,417 and 3,816,223. Some improvement was realized, butthe opaque mask material was still prominently positioned and thussusceptible to injury. Further improvement was therefore desired.

An object of this invention, therefore, is to provide a photomask thatis more resistant to damage than those photomasks heretofore known andto provide a method to make the subject mask.

SUMMARY OF THE INVENTION

This invention is characterized by a photomask comprising a glasssubstrate with depressions in selected regions of one surface thereofwith an opaque ceramic glaze or other masking material fused into thedepressions. The thickness of the glaze masking material is less thanthe depth of the depressions and only a narrow peripheral portion aroundeach region of masking material is coplanar with the glass surface. Theremainder of the surface of the masking material is depressed withrespect to the major surface of the mask. Thus, the opaque maskingmaterial is less likely to be scratched, and in the event that the hardglass substrate of the subject mask becomes scratched, the mask canoften be restored by lapping the major surface to remove the damage fromthe substrate.

The subject photomask is manufactured by positioning a metal stencil onone of the major surfaces of a glass substrate such that onlypreselected areas of the substrate are exposed. A grit etching stepfollows to form depressions in the preselected areas. Next, a ceramicmasking material, such as a fusible ceramic glaze, is sprayed into thedepressions. The metal stencil is then removed. Finally, the combinationis heated to fuse the masking material to the substrate. Following thefusing step, it may be preferable to lap and polish the major surface toassure that the peripheral portions of the masking material are coplanarwith the major surface of the substrate.

DESCRIPTION OF THE DRAWINGS

These and other features and objects of the present invention willbecome more apparent upon a perusal of the following description takenin conjunction with the accompanying drawings wherein:

FIG. 1 is a sectional elevation view of a portion of a glass substratewith a metal stencil positioned thereon;

FIG. 2 is a view similar to FIG. 1 wherein the stacked combination ofFIG. 1 has been placed on a clamping magnet and support and subjected toa grit etching step;

FIG. 3 illustrates the combination of FIG. 2 following the deposition ofparticulate masking material thereon;

FIG. 4 is an elevation view of the subject photomask following theremoval of the metal stencil and the subsequent fusing of the ceramicglaze masking material to the substrate; and

FIG. 5 is a sectional elevation view of a lapped, finished photomask inaccordance with the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD

Referring first to FIG. 1, the fabrication of the subject photomask willbe described. A glass substrate 11 defining two smooth, substantiallyparallel major surfaces 12 and 13 is provided. The glass 11 ispreferably optical quality photographic plate glass of the type that iscommercially available from companies such as the Eastman Kodak Companyof Rochester, N.Y. A metal stencil 14 is positioned on the surface 12 asillustrated in FIG. 1. The stencil defines openings which leave exposedpreselected areas 15 of the major surface 12. It should be realized thatthe metal stencil is essentially a negative of the preselected areas 15that are to be rendered opaque in the finished photomask.

The results of the next following step are illustrated in FIG. 2. Thepreselected areas 15 are grit etched to form depressions 16 in the majorsurface 12. The grit etching is performed with commercial grit etchmachines which are commonly used in semiconductor manufacture. It hasbeen found that a good mask will be provided if the depressions are inthe range of 2 to 4 mils deep. Furthermore, it is preferable to gritetch with as fine a material as practicable, such as with 400 meshaluminum oxide grit.

FIG. 2 also illustrates an advantageous method of maintaining thestencil mask in its proper position. If the metal stencil 14 is composedof a ferromagnetic material and the glass substrate 11 and stencil 14are placed in a stacked relationship on a clamping magnet support 17,the metal stencil will be firmly held in position.

If desired, the stencil 14 can now be temporarily removed to facilitatecleaning the grit medium and other detritus from the substrate 11. Ifthe mask 14 is removed, care must be taken to assure precise realignmentbefore proceeding with the following steps.

Fusible optical masking material 18 is deposited in the depressions 16as illustrated in FIG. 3. The fusible masking material 18 canadvantageously be sprayed into the depressions and can consist of aceramic material such as a ceramic glaze material compatible with theglass substrate. It, advantageously, can be a commercially availableglass decorating glaze material in a particulate form dispersed in avolatile carrier liquid. For example, Matthey-Bishop V-2806 Black GlazePowder can be employed as the optical masking material. The opticalmasking material must be opaque when fused, but only necessarily opaquewith respect to the particular radiation which is intended to be used topolymerize the photoresist during processing utilization of the subjectmask.

A preferable method of depositing the fusible glaze material is byconventional wet spray application techniques. For instance, theparticulate glaze material is mixed and diluted with the carrier liquidas necessary to facilitate its use in commercially available sprayingapparatus. For example, when using a Paashe Airbrush with a No. 1 SprayTip, the glaze can advantageously be prepared by mixing 48 grams of theglaze powder with 12 milliliters of Matthey-Bishop G-4134 ScreeningMedium and thoroughly mixing the resulting material with fourmilliliters of xylene. Finally, the glaze is diluted to 100 milliliterswith clean trichlorethylene. The mixture is then sprayed through themask into the depressions 16 by conventional airbrush techniques. Next,the stencil 14 is removed and the substrate 11 is removed from themagnetic body 17.

A heating step which fuses the optical masking material to the substrateand into a coherent body follows. Preferably, the heating step comprisesa prebake step to drive off the liquid carrier. Preferably, the assemblyis air-dried for 2 hours, baked out for 2 hours at 110° C., andthoroughly baked again at 210° C. for 12 or more hours. Subsequently,the final heating step is carried out for a time and temperaturecommensurate with the fusion characteristics of the particular maskingmaterial.

The final heating step is preferably accomplished in a tunnel oven setto provide a slow rise of about 1/2hour to the fusion temperature, and aslow cool of about 1 hour from the fusion temperature. A time of 15minutes or more at fusion temperature in the range of 400° to 650° C. istypical. The precise temperature and time used to fuse the glaze into acontinuous pinhole-free surface and to bond the glaze to the glasssubstrate will depend on the masking material 18 used. For example, whenusing the Matthey-Bishop compound cited, a temperature of 500° to 520°C. for a period of approximately 15 minutes to 1 hour has been found togive the desired result. Preferably, the glaze firing temperature is nogreater than the annealing temperature of the glass substrate.

As is shown most clearly in FIG. 4, which illustrates the mask followingremoval of the stencil 14 and the heating step, the optical maskingmaterial 18 forms a meniscus around the periphery of the depression.Furthermore, as shown in FIG. 4, small particles 19 of the maskingmaterial may inadvertently come in contact with and be fused to themajor surface 12 in areas removed from the depressions 16. Clearly,these small opaque particles are deleterious to the operation of themask. Thus, it is helpful to subject the major surface 12 to a lappingand polishing step following the fusing heat treatment. Preferably, thesubject mask is lapped to approximately the level of the broken lineA--A in FIG. 4. The lapping is by conventional procedures such thatapproximately 1/2 to 1 mil of material is removed from the major surface12.

A finished photomask 21 made in accordance with the subject invention isshown in FIG. 5. It will be appreciated from an observation of FIG. 5that the depressions 16, even after the major surface 12 has been lappedto the final level, denoted 12', are still deeper than the fused,densified masking material 18 is thick. Thus the surface of the maskingregion, except for a narrow peripheral portion 22 of each region, isdepressed with respect to the surface 12'. The only area of the surfaceof each body of optical masking material that is coplanar with respectto the surface 12' is the narrow peripheral portion 22 which serves as alight sealing rim and thus assures that when the surface 12' is placedadjacent photoresist to be exposed, a reliable light seal is createdaround the desired periphery of the masked area. It will further beappreciated that the junction between the glass substrate 11 and theoptical masking material 18, where exposed, is substantiallyperpendicular to the surface 12', thus minimizing refraction andreflection problems and further assuring accurate exposure of thephotoresist. Furthermore, the perpendicularity of the substrate-maskingmaterial interface 23 to the surface 12' assures the minimization ofdiffraction effects.

Many process modifications are within the scope of this invention. Forexample, if the heating step utilized to fuse the glazing material tothe substrate is approximately at the annealing temperature of theglass, it is helpful to aneal the substrate prior to the grit-etchingstep. Photographic glasses of the type cited have been found to annealat approximately 515° to 525° C. The temperature required to fuse theglaze is determined by the chemical composition and is ordinarilycontained in the information supplied by the gaze manufacturer. In theabove example, uitilizing the Matthey-Bishop Glaze, it is helpful toanneal the glass substrates on a flat plate for a time sufficient torelieve internal stresses, generally about an hour, prior to gritetching. It is also useful to lap the first major surface 12 followingthe annealing process but prior to the grit-etching step so that themetal stencil will form a better seal.

Another useful process modification is to lightly chemically etch thesubstrates in a buffered hydrofluoric acid, following the grit etch. Theacid will remove any small clinging particles of glass that were notcompletely removed by the grit etch. The chemical etch will furthereliminate much of the surface damage caused to the substrate by the gritetch. It will be appreciated, of course, that if the chemical etch isdesired, the metal stencil 14 may be temporarily removed and thenreplaced prior to the spraying of the optical masking glaze coating.

Other glass substrates can be used. If a low temperature glaze isdesired, a low temperature glass which handles well and anneals at about300° C. could be used. Or, if a higher temperature glaze is used, it canbe used with a higher temperature substrate such as fused quartz whichanneals at about 1000° C. or more and shows good transmission in theultraviolet-visible spectrum.

Another advantage of the mask manufactured as described above is thatthe bottoms of the depressions 16, being formed by grit etching, arerelatively rough, even when etched as described above, and thus theopaque areas, when viewed from the surface 13 have a matte, rather thanglossy finish. Thus, when the mask is illuminated from the surface 13during use, light is scattered, rather than reflected, by the opaqueareas. Therefore, inadvertent exposure of photoresist is less likely tooccur. The matte finish effect is enhanced if some voids remain betweenthe bottom of the depressions and the mask material 18.

Many other modifications and variations of the subject invention will beapparent to those skilled in the art. For example, the masking materialcan be rolled or brushed into the depressions. It is to be understood,therefore, that the invention can be practiced otherwise than asspecifically described.

What is claimed is:
 1. A photomask comprising a glass substrate havingdepressions formed in a major surface thereof and opaque ceramic maskingmaterial fused to said substrate in said depressions, wherein only anarrow peripheral portion of each body of said masking material iscoplanar with said major surface, the remaining portion of the exposedsurface of each of said bodies of masking material being depressedbeneath the plane of said major surface.
 2. A photomask according toclaim 1 wherein the marginal portions of the junctures of said substrateand said bodies of masking material are substantially perpendicular tosaid major surface.
 3. A photomask according to claim 1 wherein thebottoms of said depressions are relatively rough.